1. Field of the Invention
The present invention provides a flexible elastic airfoil section that adapts its shape to loading requirements, and a finite wing or blade made up of such airfoil sections, that obtains a substantial portion of its lift from cambered deflections. Such an airfoil section has application in a variety of subsonic aerodynamic and hydrodynamic applications whenever a wing or blade is required to produce both positive and negative loads as well as to generate a wide range of forces with good aerodynamic efficiency. The invention can be used to stabilize or control the direction of travel of an aircraft or a watercraft. It can be used to provide lift for an aircraft or sideforce for a sailing craft. It has application in the design of a wide variety of aircraft components and of various aerodynamic devices for fluid machinery.
2. Description of the Prior Art
The present invention deals with the design of airfoil sections and wings for a variety of applications. As outlined herein, the principles of design of airfoils and wings for operation in a subsonic flow of a gas such as air are the same principles used to design foil sections for application to incompressible fluids such as water. Therefore the present disclosure will deal with the design and operation of airfoil sections and wings in both gases and incompressible fluids and the application of these devices to aircraft, watercraft and fluid machinery. The general term fluid will be used to refer to either a gas or a liquid. The term airfoil or its equivalents, airfoil section and foil, will be used to describe a streamlined shaped profile designed for operation in either a gas or a liquid. The term wing will be used to describe a structure of finite span whose cross-sections are airfoil sections designed for operation in a gas such as air, where it may serve as an aircraft wing, a vertical stabilizer or other aircraft control surfaces, or such a structure designed for operation in a liquid such as water where it may serve as a keel, centerboard or rudder for a watercraft, or a fin for a sailboard. The present invention may also be applied in the design of blades for rotating machinery in air, such as a fan, a wind turbine or a helicopter rotor, or for rotating machinery in a liquid, such as a propeller in water or for general applications in the design of fluid machinery.
The lift on an airfoil section is defined as the force in the plane of the section perpendicular to the oncoming flow direction; the drag on an airfoil section is defined as the force in the plane of the section in the direction of flow. Airfoil sections have a wide variety of applications: providing lift to an aircraft, providing sideforce to a stabilizing device such as the centerboard of a watercraft, and obtaining thrust and transferring power to or from a blade in a fan, wind turbine or propeller. In all of these applications, the forces provided by the various devices, the wing, the centerboard, and the blade are derived from the lift and drag of the individual airfoil sections. Therefore, in the disclosed invention, we will refer to the forces generated by the airfoil sections as lift and drag forces even though in application these give rise to sideforce and thrust as well as lift and drag depending on the geometry of the application.
It is well known that, to achieve efficient subsonic aerodynamic performance at a given design point, airfoil sections should be designed with a combination of camber (curvature of the airfoil centerline) and angle of attack (attitude to the wind). Modern high-lift airfoils obtain comparable portions of their total lift from camber and angle of attack. A well designed cambered airfoil section will produce the same lift with a lower drag penalty than a symmetric airfoil section.
This presents a problem to the designer of an airfoil section for a craft which must operate in a symmetric manner, such as a sailboat which must operate equally well on both tacks. For such applications, the predominant prior-art airfoil designs have been of two types: rigid symmetric airfoils, such as are used for keels, rudders, vertical stabilizers, and centerboards; and flexible snap-through camber devices such as sails, which have an excess material with little inherent stiffness that assumes a curved shape under load, or thick airfoils with flexible skins, which have a pre-determined shape under load due to internal structure and linkages. When an angle of attack of operation is selected and load is generated, both of these devices snap through to a predetermined fixed non-symmetric aerodynamic shape, providing some load due to the curvature or camber of this excess material or non-symmetric shape.
For these airfoils, additional increments of load are provided primarily by changes in angle of attack and this load is accompanied by strong negative pressure peaks near the leading edge of the airfoil. If operation at high lift coefficients is required, such airfoils are prone to stall and, for operation in water, to ventilation and cavitation. It is an aspect of the present invention to provide devices which are resistant to stall and ventilation.
Such snap-through airfoil section designs operate poorly at zero angle of attack where the lift force is not sufficient to force them to take their designed cambered shape. For a sail with excess material, operation at low angles of attack produces flutter and vibration; for a flexible airfoil section with internal linkages, it is likely that low lift will be accompanied by vibration leading to high drag and fatiguing of mechanical structures.
There exist many prior art devices which achieve the benefits of lift due to camber by employing variable camber and/or changes in the cross-sectional shape of the airfoil sections. Aircraft wings are designed with a plurality of rigid sections, such as leading and trailing edge flaps, which are deployed as required to increase the effective camber of the airfoil, thereby providing increased lift. Most airfoil sections designed for aircraft are not required to operate symmetrically at both positive and negative angles of attack producing positive and negative loads. Rigid airfoils for aircraft wings can be designed with initially cambered centerlines. Additional changes in camber can be accomplished with actuating machinery, say by lowering the leading and trailing edge flaps. This requires intervention of the pilot or an active control system.
It is an aspect of the present invention that this change in airfoil section geometry is accomplished passively, that is without employing any actuating machinery in response to the aero/hydrodynamic forces on the airfoil section, and that this change in airfoil geometry occurs equally well for positive and negative load.
For watercraft, several designs for variable camber devices have been proposed wherein the effective camber shape is adjusted by pumping fluid or causing fluid to be pumped into flexible reservoirs on either side of a rigid central surface, or otherwise deploying flexible surfaces on either side of a rigid support structure, often with a variety of internal linkages. Such designs are extremely complex and limit the benefits of camber that can be achieve because of the rigid central section which constrains the travel of the foil centerline.
Each of the prior art, variable-camber devices share several of the following characteristics that differ from the present invention: the flexible surface is deployed about a central rigid structure located on the undeformed centerline of the airfoil section, limiting the deflection of the airfoil under load; the deflection of the airfoil is not proportional to load over a substantial range of load, so that it deflects to a fixed limiting shape at small loads; the surface of the airfoil is discontinuous, with layers of the skin free to slide, forming discontinuities in the surface slope; the flexibility of the airfoil is not chosen with a relationship to the dynamic pressure of the flow. It is an aspect of the present invention to overcome these limitations of prior art devices.
There are unresolved questions regarding the aeroelastic stability of prior art devices; for sufficiently high flow speeds, such devices will become unstable and behave in an uncontrollable manner. Such instabilities are of two types. The first is static aeroelastic divergence, where the flexible shape deforms uncontrollably. These high deflections can lead to undesirable aerodynamic shapes, leading to high drag forces. The second instability is flutter, an unstable vibratory motion which will occur for a flexible surface operating at sufficiently high speeds relative to the stiffness of the flexible surface. Flutter can lead to high drag, fatigue of mechanical surfaces, and catastrophic failure of the airfoil system. The designer of flexible devices for use in a flowing medium must take such instabilities into account when choosing the elastic properties of the flexible surfaces.
The present invention also has application in the design of various aircraft components such as helicopter, wind turbine and fan blades. These blades are currently designed with rigid airfoil sections. The performance range of such devices is limited by the stall of the blade sections. The airfoils described here postpone stall to higher loadings while having good aerodynamic performance at light loading where the blade chamber will be small. Other applications include aircraft horizontal stabilizer and rudder surfaces; application of these airfoils leads to enhanced aircraft stability and control authority in comparison to the current rigid control surfaces. Alternatively, smaller control surfaces of the same effectiveness as large rigid surfaces can be used resulting in reduced drag.